The invention relates generally to cell selection by a mobile station in a cellular radio network where cells can be defined as subscriber-specific special cells.
In cellular mobile communication systems a mobile station can move freely in the area of the mobile communication network and camp on the most suitable cell at each time. When the mobile station (MS) is switched on, it attempts to make contact with a public land mobile network (PLMN). A particular PLMN to be contacted can be selected either automatically or manually. The MS searches for a suitable cell of the selected PLMN and selects that cell to provide the available services and tunes onto the control channel (broadcasting channel) of the selected cell. The selection is referred to as camping on a cell. For the purpose of cell selection, the MS measures all radio channels of the PLMN or all radio channels according to a stored cell list. The MS typically calculates for each radio channel several averages of the measured received signal level used for cell selection. The MS can then also register its presence in the location area (LA) of the selected cell if necessary, by a location updating procedure or the like. When the MS has carried out the cell selection process and selected the cell, it starts to listen to the control channel of this cell to receive services. The cell that the MS is camped on in this way is called a serving cell. When the MS is camped on a cell but does not communicate with the cell (no call), it is in a mode called an idle mode. In the idle mode the MS measures not only the signal received from the serving cell, but at least the received signal levels of non-serving adjacent cells. An average is generally calculated from the measured signal levels of each cell, the average being used as a cell selection criterion as such, or by calculating specific cell selection parameters. The MS may also monitor the radio path loss between the mobile station and the serving base station and reject in the selection process the cells where the radio path loss is lower than a specific threshold value. Furthermore, cell selection criteria may include hysteresis and timings by which (in addition to averaging measurement results) unnecessary cell reselections and location updatings due to temporary variations in radio conditions are to be prevented. As examples of the camping facility and cell selection process and criteria of the mobile station, reference is made to GSM (Global System for Mobile Communications) and DCS (Digital Communication Systems) mobile communication systems, particularly to the GSM recommendation GSM 03.22 (version 5.0.0) and 05.08 (version 5.1.0).
One feature of cellular radio networks is that the network should know the location of the mobile station in the network to be able to page it and route calls to it. Logical location areas consisting one or more cells are typically defined in cellular networks. The information about the location of the mobile station is stored in the network at the accuracy of a location area. Cells broadcast information indicating the location area they belong to. When the mobile station upon selecting a new cell finds that the location area is changing, it makes a location updating to the network. In order to reduce location updating signalling it would be advantageous that the location areas were as large as possible. On the other hand, the mobile station is paged in all the cells of the location area, in which case it would be preferable for paging signalling that the size of the location area would be as small as possible. The size of the location area is typically a compromise between these two demands.
One known alternative is that instead of or in addition to fixed location areas, location areas specific for each subscriber are determined in the network, and cells are selected to the subscriber-specific location areas to the effect that the typical mobility area of a specific subscriber and other requirements are taken into account as well as possible.
One of the objectives in mobile communication systems has recently been to develop mobile station services to be more and more equal to the conventional services of the fixed network so that they could replace fixed-line services both in offices and at home. In other words, a subscriber or a company is provided with a service area to which specific cells are selected (known e.g. as special cells) where a local subscriber-specific service profile, for example, is defined. That is to say, services and tariffs dependent on the location of the subscriber are produced for the subscriber. This cluster of cells is referred to as a Localized Service Area LSA or an LGS area. The subscriber can be an ordinary home user, for example, whose LSA is part of the radio access network covering his/her home and its immediate surroundings. Within this LSA special tariffs can be offered only for the home subscriber. The subscriber can also be a company employee to whom the localized service area LSA forms a company-wide network that utilizes the radio access solutions in the premises, for example. In the LSA, special tariffs can be offered only for company employees. The LSA typically comprises a cell or a cluster of cells. The cells in the cluster of cells can also be dispersed.
It would be advantageous for the user to be able to use the special cells and the special tariffs and services provided by them as much as possible. As described above, in present cellular network systems the mobile station selects a cell with algorithms based on signal strength. In these algorithms all cells are treated democratically, that is, the selection can be directed to any suitable cell. When the subscriber of the LSA service is a home subscriber, the LSA is home, a flat or a house; a very compact area in any case. In that case the cell that is the dominating cell in this location area is selected as the subscriber""s xe2x80x9chome cellxe2x80x9d. It can be assumed that mobile calls in this location area are served by the home cell, but this cannot be ensured because of the fading phenomenon associated with radio wave propagation, for example. This can lead to that the home subscriber is not always able to utilize a lower tariff or special services. Even if the mobile station was provided with a feature that would inform the subscriber if the subscriber is camped on a special cell or not, the use of the service would be uncertain and uncomfortable for the subscriber. Correspondingly, the LSA of a company subscriber could typically be an office or a factory area. The LGS service is provided by means of a base station placed in the company premises. The coverage area of the base station is planned so that the LSA cell covers the whole defined LSA, but it does not necessarily dominate the whole area. For example, at the boundaries of the LSA cell, such as near room windows, the downlink signal of some adjacent cell can be stronger, in which case under normal conditions the MS camps on this adjacent cell. If the office is small and office-specific cells are not installed there, the office cell is similar to the xe2x80x9chome cellxe2x80x9d concept described above.
The object of the invention is to improve cell selection in the cellular radio network to the effect that the mobile station prioritizes the special cell/cells whenever it is in its/their area.
This will be attained with the method of the invention for selecting a cell of a cellular radio network in a mobile station, which method comprises the steps of:
measuring signal levels of a serving cell and adjacent cells in a mobile station in the idle mode,
calculating a cell selection parameter for each measured cell by means of the measured signal levels, and
selecting the best cell to be the serving cell on the basis of the cell selection parameters.
The method is characterized in that the method further comprises the steps of:
comparing the cells to be measured with a special cell list stored in the memory of the mobile station,
checking if each cell identified as a special cell fulfils the pre-determined minimum requirement of cell selection on the basis of the measured signal level, and
manipulating the calculation of a cell selection parameter of a special cell that fulfils the minimum requirement to the effect that the selection probability of a special cell is improved with respect to a normal cell.
The invention further relates to a cellular radio system comprising mobile stations and base stations, the mobile stations being arranged to measure in the idle mode the received signal levels of cells and to select as the serving base station the cell which is the best on the basis of a cell selection parameter calculated by means of the measured signal levels. The system is characterized in that the mobile station comprises means for storing a special cell list, means for comparing the cells to be measured with the special cell list, a means which checks if a cell identified as a special cell fulfils the pre-determined minimum requirement of cell selection on the basis of the measured signal level, and means for manipulating the calculation of the cell selection parameter of a special cell that fulfils this minimum requirement to the effect that the selection probability of a special cell is improved with respect to a normal cell.
The invention also relates to a mobile station which is arranged to measure in the idle mode the received signal levels of cells and to select as the serving base station the cell which is the best on the basis of the cell selection parameter calculated by means of the measured signal levels. The mobile station is characterized in that the mobile station comprises means for storing a subscriber-specific special cell list, means for comparing the cells to be measured with the special cell list, a means which checks if a cell identified as a special cell fulfils the pre-determined minimum requirement of cell selection on the basis of the measured signal level, and means for manipulating the calculation of the cell selection parameter of a special cell that fulfils the minimum requirement to the effect that the selection probability of a special cell is improved with respect to a normal cell.
In the invention the mobile station is arranged to store a list of special cells which are defined for it in the whole network or in a local section of the network. On the basis of this stored information and system information broadcast by the cells, the mobile station is able to identify the special cells in the neighbourhood.
The mobile station measures the reception levels of downlink signals of surrounding cells, including a possible serving cell (typically by calculating the average signal level for each cell on the basis of several measurements). A cell selection parameter of the used cell selection criterion is calculated by means of this measured (preferably average) signal level. In this application, the calculation of the cell selection parameter refers to the determination or calculation of any reference value used in cell selection. This parameter can be directly measured (preferably average) signal level, but typically it is another value calculated by means of the measured signal level, such as cell selection parameter C2 in the GSM mobile communication system. According to the basic principle of the invention, the calculation of the cell identified as a special cell is processed to the effect that the special cell will be provided with a better cell selection parameter value than a normal cell on the same measured signal level. In this way the selection probability of the special cell is improved in comparison with the normal cell. In other words, the cell selection algorithm of the invention prioritizes special cells with respect to normal cells. In the invention the prioritization is made only if the special cell fulfils the pre-determined minimum requirement of cell selection on the basis of the measured signal level. In practice this may mean that the received signal level has to exceed the predetermined minimum level, or the radio path loss between the mobile station and the base station has to fall below a certain threshold value. For example, in the GSM system the path loss parameter C1 has to be higher than zero before the special cell prioritization of the invention is allowed. In this way the prioritization of the special cell is prevented from causing the mobile station to be camped on a special cell whose signal level is too low, and thus the negative effects on the spectrum efficiency of the mobile network are avoided. Operators of the mobile network occasionally set the minimum requirement of the cell selection of normal cells very low. In such a case a higher minimum requirement of cell selection can be used for special cells than for normal cells and this way the selection of a special cell of a very poor quality can be avoided. The minimum requirements of cell selection are preferably selected by the operator and the cells typically broadcast information reporting the minimum requirements.
In the preferred embodiment of the invention, a special cell is prioritized by adding a pre-determined margin to the measured (preferably average) signal level of the special cell before the actual determination of the cell selection parameter and completion of the cell selection. In this way, the xe2x80x9cmeasuredxe2x80x9d signal level used in the cell selection is higher than in reality, which naturally leads to that the special cell is classified as the best or as one of the best in cell selection. In this way the invention ensures very efficiently that a special cell is selected as a serving cell provided that it has a sufficient signal level. The use of the margin of the invention makes it, however, possible for the mobile station to camp on a normal cell as well in case the signal strength of the normal cell exceeds the signal strength of the special cell by at least the margin mentioned above. This stops the mobile station from xe2x80x9changing ontoxe2x80x9d the special cell for ever as would be the case if the cell selection took place only when the signal level falls below the minimum requirement. By suitable selections of the margin and minimum requirement of the invention, a required balance can be looked for between the use of the special cell and the neighbouring adjacent cells without deteriorating the service level or without causing negative effects to the spectrum efficiency of the network. This margin is preferably broadcast to the mobile stations from the base station of the serving cell so that it can be adjusted dynamically and locally.
Alternatively, a corresponding margin can be added directly to the final cell selection parameter or any other parameter used in the calculation of the cell selection parameter.